1. Field of the Invention
The present invention relates to oscillators used as oscillation mixer apparatuses including negative differential resistance elements for generating electromagnetic waves. The present invention particularly relates to an oscillator which oscillates electromagnetic waves in a frequency band within a frequency region from a millimeter waveband to terahertz waveband (equal to or larger than 30 GHz to equal to or smaller than 30 THz).
2. Description of the Related Art
As has been broadly recognized, a negative differential resistance element is useful in an application field of oscillators which generate electromagnetic waves when being used with a resonator. The negative differential resistance element generates electromagnetic waves (hereinafter simply referred to as terahertz waves) including at least a number of frequency components in the frequency band in the range from the millimeter waveband to the terahertz waveband (equal to or larger than 30 GHz to equal to or smaller than 30 THz). As an example, an oscillator including a negative differential resistance element on a substrate in a monolithic manner is disclosed in “IEEE Electron Device Letters, Vol 18, 218 (1997)”. A slot antenna is disposed on the semiconductor substrate including the negative differential resistance element in an integration manner and a resonance configuration and a gain medium are configured in a monolithic manner.
FIG. 10 is a diagram illustrating an oscillator disclosed in “IEEE Electron Device Letters, Vol 18, 218 (1997)”. This oscillator includes a resonant-tunneling diode S-RTD 11 having a Schottky barrier on a collector side as a negative differential resistance element. As a resonator, a slot antenna is used. The slot antenna according to “IEEE Electron Device Letters, Vol 18, 218 (1997)” includes a metallic pattern 12 disposed on a semiconductor substrate, and further includes capacitors 13 and 14 at opposite ends of the pattern. The oscillator according to “IEEE Electron Device Letters, Vol 18, 218 (1997)” further includes a rectifier diode 15. Here, the rectifier diode 15 functions as a stabilizing circuit used to suppress parasitic oscillation which causes a problem in oscillators employing negative differential resistance elements. The parasitic oscillation means oscillation which is parasitically generated in a low frequency band and which is different from a desired frequency. Since such parasitic oscillation considerably deteriorates an oscillation output in a desired frequency, the presence of stabilizing circuits in oscillators employing negative differential resistance elements is considerably important. A detailed configuration will be described below. Assuming that an oscillation wavelength of an oscillator is denoted by λosc and an oscillation frequency is denoted by ωosc, in order to suppress the parasitic oscillation, an impedance of a power source used to supply bias should be low in a frequency region in a range equal to or larger than DC and smaller than ωosc. Accordingly, as one method, a low impedance circuit (such as a shunt rectifier diode) should be disposed in a position within λosc/4 on the power source side relative to an S-RTD. Therefore, in FIG. 10, as such a low impedance circuit, the rectifier diode 15 is disposed in a position within λosc/4 on a power source 16 side relative to an S-RTD 11 in an integration manner. A resistance 17 represents the sum of an internal resistance of the power source 16 and a resistance included in a connection line.
Furthermore, by changing a capacitance component included in a resonator, an oscillator may have a variable oscillation frequency and variable amplitude. Japanese Patent No. 3923260 discloses a variable oscillator employing a Schottky diode disposed on a substrate including a negative resistance diode disposed thereon in an integration manner as a varactor (variable capacitance element). A capacitance of the varactor can be controlled by a power source disposed separately from a power source used to apply bias on the negative resistance diode. Therefore, the variable oscillator disclosed in Japanese Patent No. 3923260 can be used as a voltage controlled oscillator (VCO) to transmit millimeter waves which are oscillated and modulated by controlling the bias of the power source.
However, in the oscillators according to the related art, even if a control bias is modulated at ultrahigh speed, the oscillators may not follow the ultrahigh-speed modulation. This is because a stabilizing circuit should be inserted between such an oscillator and a power source in order to suppress parasitic oscillation unique to a negative differential resistance element. In this case, the stabilizing circuit includes a shunt resistance, for example, which is a comparatively low resistance. Taking this resistance and a parasitic inductance in a connection line which connects the power source and the stabilizing circuit to each other into consideration, a low-pass filter circuit is configured. That is, a modulation signal corresponding to a high frequency and ultrahigh speed is filtered by the stabilizing circuit. Therefore, negative differential resistance elements are not allowed to be modulated at ultrahigh speed in the oscillators according to the related art.